1. Field of the Invention
The present invention relates to an internal combustion engine and, in particular, relates to an internal combustion engine that includes a balance shaft housing that is disposed between a crankcase and an oil pan and provided with a balance shaft in a rotatable manner.
2. Description of Related Art
An internal combustion engine that includes a balance shaft housing disposed between a crankcase and an oil pan has been known (see Japanese Patent Application Publication No. 2012-007691 (JP 2012-007691 A), for example).
Generally, in a multi-cylinder internal combustion engine, when rotational imbalance of a crankshaft is caused by a secondary inertia force that is generated by a vertical reciprocating motion of a piston, this further causes vertical vibration of the internal combustion engine. Accordingly, the internal combustion engine is provided with a balancer device to suppress the vertical vibration.
The internal combustion engine that is disclosed in JP 2012-007691 A includes a crankcase, an oil pan that is fastened to a lower part of the crankcase, and a balancer device that is disposed between the crankcase and the oil pan.
The balancer device includes two balance shafts that are rotationally driven according to rotation of the crankshaft as well as an upper housing and a lower housing that house the balance shafts in a rotatable manner. Each of the balance shafts is configured such that the center of gravity thereof is deviated from the center of rotation thereof, and the vertical vibration and the like that are caused by the secondary inertia force of an engine can be suppressed by the rotation of the balance shafts.
Meanwhile, a relatively large opening has to be formed in a bottom surface of the crankcase in order to dispose an oil pump in the crankcase, install the balance shafts, and secure an amount of oil. The formation of the opening in the bottom surface of the crankcase causes twisting, bending, or the like of the crankcase itself. Consequently, rigidity of a power plant that is formed with the engine, a transmission, and the like is decreased.
In addition, due to a decrease in the rigidity of the power plant, vibration of an engine mount is increased, and consequently, the engine noise becomes louder. Meanwhile, in order to secure the rigidity of the power plant, it is considered to dispose a rib or the like in the rear (a side part where a speed changing mechanism such as the transmission is disposed) of the crankcase for reinforcement.
However, there is space limitation in the rear of the crankcase for disposing the rib or the like because a space for fastening a drive plate and the like of the transmission is necessary in the rear of the crankcase. In addition, if the rib and the like are disposed to reinforce the crankcase, it causes a disadvantage such as an increase in a mass of the crankcase.